U.S. patent number 10,070,076 [Application Number 15/830,064] was granted by the patent office on 2018-09-04 for drift correction method for infrared imaging device.
This patent grant is currently assigned to CI SYSTEMS (ISRAEL) LTD.. The grantee listed for this patent is CI SYSTEMS (ISRAEL) LTD.. Invention is credited to Dario Cabib, Moshe Lavi, Liviu Singher.
United States Patent |
10,070,076 |
Cabib , et al. |
September 4, 2018 |
Drift correction method for infrared imaging device
Abstract
A method reduces drift induced by environment changes when
imaging radiation from a scene in two wavelength bands. Scene
radiation is focused by two wedge-shaped components through a lens
onto a detector that includes three separate regions. The
wedge-shaped components are positioned at a fixed distance from the
lens. The radiation from the scene is imaged separately onto two of
the detector regions through an f-number of less than approximately
1.5 to produce a first pixel signal. Imaged radiation on each of
the two regions includes radiation in one respective wavelength
band. Radiation from a radiation source is projected by at least
one of the wedge-shaped components through the lens onto a third
detector region to produce a second pixel signal. The first pixel
signal is modified based on a predetermined function that defines a
relationship between second pixel signal changes and first pixel
signal changes induced by environment changes.
Inventors: |
Cabib; Dario (Timrat,
IL), Lavi; Moshe (Nofit, IL), Singher;
Liviu (Kiryat Tivon, IL) |
Applicant: |
Name |
City |
State |
Country |
Type |
CI SYSTEMS (ISRAEL) LTD. |
Migdal Ha'emek |
N/A |
IL |
|
|
Assignee: |
CI SYSTEMS (ISRAEL) LTD.
(Migdal Ha'emek, IL)
|
Family
ID: |
61685912 |
Appl.
No.: |
15/830,064 |
Filed: |
December 4, 2017 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20180091749 A1 |
Mar 29, 2018 |
|
Related U.S. Patent Documents
|
|
|
|
|
|
|
Application
Number |
Filing Date |
Patent Number |
Issue Date |
|
|
14949906 |
Nov 24, 2015 |
|
|
|
|
62088720 |
Dec 8, 2014 |
|
|
|
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
H04N
5/2353 (20130101); G01J 5/0014 (20130101); H04N
5/33 (20130101); H04N 5/357 (20130101); H04N
5/3655 (20130101); G01J 5/0803 (20130101); H04N
5/217 (20130101); G01J 5/52 (20130101); G01J
2005/0077 (20130101); G01J 2005/0048 (20130101) |
Current International
Class: |
H04N
5/33 (20060101); G01J 5/00 (20060101); H04N
5/365 (20110101); H04N 5/357 (20110101); G01J
5/08 (20060101); G01J 5/52 (20060101); H04N
5/235 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
Primary Examiner: Porta; David
Assistant Examiner: Valentiner; Jeremy S
Attorney, Agent or Firm: Friedman; Mark M.
Parent Case Text
CROSS-REFERENCE TO RELATED APPLICATIONS
This application is a continuation of U.S. patent application Ser.
No. 14/949,906, filed on Nov. 24, 2015, now issued as U.S. Pat. No.
9,876,968, which claims priority from U.S. Provisional Patent
Application No. 62/088,720, filed Dec. 8, 2014. This application is
related to the commonly owned U.S. patent application Ser. No.
14/949,909, filed on Nov. 24, 2015, now issued as U.S. Pat. No.
9,759,611. All of the disclosures of the aforementioned
applications are incorporated by reference in their entirety
herein.
Claims
What is claimed is:
1. A method for reducing drift induced by at least one changing
environment feature when imaging radiation from a scene, the method
comprising: (a) focusing, over a duration of time, radiation from
the scene through an image forming optical component onto a first
region of a detector and a second region of the detector to produce
at least a first pixel signal; (b) projecting radiation from a
radiation source through the image forming optical component onto a
third region of the detector separate from the first and second
regions of the detector to produce a second pixel signal, the
radiation source being different from the scene, and the radiation
from the radiation source being continuously projected onto the
third region of the detector over the duration of time for which
the radiation from the scene is focused onto the first and second
regions of the detector; and (c) modifying the first pixel signal
based in part on a predetermined function to produce a modified
pixel signal, the predetermined function defining a relationship
between a change in the second pixel signal and a change in the
first pixel signal induced by the at least one changing environment
feature.
2. The method of claim 1, wherein the image forming optical
component is positioned within a first enclosure volume.
3. The method of claim 2, further comprising: (d) positioning the
radiation source proximate to the image forming optical component,
prior to performing (b).
4. The method of claim 3, wherein the first enclosure volume is
positioned within a second enclosure volume, and wherein the
positioning the radiation source proximate to the image forming
optical component includes: (i) positioning the radiation source
within the second enclosure volume and outside of the first
enclosure volume.
5. The method of claim 1, further comprising: (d) determining the
change in the first pixel signal induced by the at least one
changing environment feature based on the predetermined function,
and wherein the modified pixel signal is produced by subtracting
the determined change in the first pixel signal from the first
pixel signal.
6. The method of claim 1, wherein the predetermined function is a
correlation between the second pixel signal and the change in the
first pixel signal induced by the at least one changing environment
feature.
7. The method of claim 6, further comprising: (d) determining the
correlation, wherein the determining of the correlation is
performed prior to performing (a).
8. The method of claim 6, wherein the radiation source is a
blackbody radiation source, and wherein the image forming optical
component is positioned within a first enclosure volume, and
wherein the detector and the first enclosure volume are positioned
within a chamber having an adjustable chamber temperature, and a
verification of the correlation is determined by: (i) measuring a
first temperature of the blackbody radiation source at a first
chamber temperature and measuring a subsequent temperature of the
blackbody radiation source at a subsequent chamber temperature, the
first and subsequent temperatures of the blackbody radiation source
defining a first set; (ii) measuring a first reading of the second
pixel signal at the first chamber temperature and measuring a
subsequent reading of the second pixel signal at the subsequent
chamber temperature, the first and subsequent readings of the pixel
signal defining a second set; and (iii) verifying a correlation
between the first and second sets.
9. The method of claim 6, wherein the radiation source is a
blackbody radiation source, and wherein the image forming optical
component is positioned within a first enclosure volume, and
wherein the detector and the first enclosure volume are positioned
within a chamber having an adjustable chamber temperature, and a
determination of the correlation includes: (i) measuring a first
reading of the first pixel signal at a first chamber temperature
and measuring a subsequent reading of the first pixel signal at a
subsequent chamber temperature; (ii) subtracting the first reading
of the first pixel signal from the subsequent reading of the first
pixel signal to define a first set; and (iii) measuring a first
reading of the second pixel signal at the first chamber temperature
and measuring a subsequent reading of the second pixel signal at
the subsequent chamber temperature, the first and subsequent
readings of the second pixel signal defining a second set.
10. The method of claim 9, wherein the modifying of the first pixel
signal includes: (i) measuring a first reading of the first pixel
signal at a first time instance and measuring a subsequent reading
of the first pixel signal at a subsequent time instance; (ii)
measuring a first reading of the second pixel signal at the first
time instance and measuring a subsequent reading of the second
pixel signal at the subsequent time instance; and (iii) subtracting
the first reading of the second pixel signal from the subsequent
reading of the second pixel signal to define a third set.
11. The method of claim 10, wherein the modifying of the first
pixel signal further includes: (iv) modifying the subsequent
reading of the first pixel signal based on the third set in
accordance with a correlation between the first and second
sets.
12. The method of claim 9, wherein the determination of the
correlation further includes: (iv) displaying the first set as a
function of the second set.
13. The method of claim 9, wherein the determination of the
correlation further includes: (iv) displaying the first set as a
function of a third set, the third set being defined by the first
chamber temperature and the subsequent chamber temperature.
14. A device for reducing drift induced by at least one changing
environment feature when imaging radiation from a scene, the device
comprising: (a) a radiation source, the radiation source different
from the scene; (b) a detector of the radiation from the scene and
of radiation from the radiation source, the detector including a
first detector region, a second detector region, and a third
detector region, the detector regions being separate from each
other; (c) an image forming optical component for focusing the
radiation from the scene onto the first and second detector regions
over a duration of time, and for continuously projecting the
radiation from the radiation source onto the third detector region
over the duration of time for which the radiation from the scene is
focused onto the detector; and (d) electronic circuitry configured
to: (i) produce at least a first pixel signal from the imaged
radiation on the first and second detector regions; (ii) produce a
second pixel signal from the radiation source projected by the
image forming optical component onto the third detector region, and
(iii) modify the first pixel signal according to a predetermined
function to produce a modified pixel signal, the predetermined
function defining a relationship between a change in the second
pixel signal and a change in the first pixel signal induced by the
changing environment feature.
15. The device of claim 14, wherein the electronic circuitry is
further configured to: (iv) determine the change in the first pixel
signal induced by the at least one changing environment feature
based on the predetermined function; and (v) subtract the
determined change in the first pixel signal from the first pixel
signal.
16. The device of claim 14, wherein the radiation source is a
blackbody radiation source.
17. The device of claim 14, further comprising: (e) a first
enclosure volume, wherein the image forming optical component is
positioned within the first enclosure volume.
18. The device of claim 17, wherein the radiation source is
positioned proximate to the first enclosure volume.
19. The device of claim 17, further comprising: (f) a second
enclosure volume, wherein at least a portion of the first enclosure
volume is positioned within the second enclosure volume, and the
radiation source is positioned within the second enclosure volume
and outside of the first enclosure volume.
20. The system of claim 14, wherein the radiation source is
positioned proximate to the image forming optical component.
Description
TECHNICAL FIELD
The present invention relates to the detection and imaging of
infrared radiation for gas cloud imaging and measurement.
BACKGROUND OF THE INVENTION
Infrared imaging devices based on uncooled microbolometer detectors
can be used to quantitatively measure the radiance of each pixel of
a scene only if the environment radiation changes (due mainly to
environment temperature changes) contributing to the detector
signals, can be monitored and corrected for. This is due to the
fact that a quantitative measurement of infrared radiation from a
scene is based on a mathematical relation between the detector
signal and the radiation to be measured. This relation depends on
the environment state during the measurement, and therefore the
quantitative scene measurement can be done only if the environment
state, and how the environment state affects that relation, is
known during the measurement. The environment radiation sensed by
the detector elements originates mainly from the optics and
enclosures of the imaging device (besides the scene pixel to be
monitored), and is a direct function of the environment
temperature. If this radiation changes in time, it causes a drift
in the signal, which changes its relation to the corresponding
scene radiation to be measured and introduces inaccuracy.
This resulting inaccuracy prevents the use of such devices,
especially in situations where they have to provide quantitative
information on the gas to be monitored and have to be used
unattended for monitoring purposes over extended periods of time,
such as, for example, for the monitoring of a scene in industrial
installations and facilities.
One known method for performing drift corrections is referred to as
Non-Uniformity Correction (NUC). NUC corrects for detector
electronic offset and partially corrects for detector case
temperature drifts by the frequent use of an opening and closing
shutter which is provided by the camera manufacturer. This NUC
procedure is well known and widely employed in instruments based on
microbolometer detectors. The shutter used for NUC is a moving part
and therefore it is desirable to reduce the number of openings and
closings of such a component when monitoring for gas leakages in
large installations, requiring the instrument to be used twenty
four hours a day for several years without maintenance or
recalibration. Frequent opening and closing of the shutter (which
is usually done every few minutes or hours) requires high
maintenance expenses.
To reduce the amount of shutter operations when using NUC
techniques, methods for correcting for signal drift due to detector
case temperature changes occurring between successive shutter
openings have been developed by detector manufacturers, referred to
as blind pixel methods. Known blind pixel methods rely on several
elements of the detector array of the imaging device being exposed
only to a blackbody radiation source placed in the detector case,
and not to the scene radiation (i.e. being blind to the scene).
However, such methods can only account and compensate for
environmental temperature changes originating near and from the
enclosure of the detector array itself, and not for changes
originating near the optics or the enclosures of the imaging
device. This is because in general there are gradients of
temperature between the detector case and the rest of the optics
and device enclosure. Therefore, known blind pixel methods may not
satisfactorily compensate for environment radiation changes in
imaging devices with large and/or complex optics, such as, for
example, optics with wedges for directing and imaging radiation
onto a detector through an objective lens system, as described
below.
SUMMARY OF THE INVENTION
The present invention is a method and device for providing a
functionality for drift correction in an infrared dual band imaging
system based on the optics described below, without the use of
moving parts.
According to an embodiment of the teachings of the present
invention there is provided, a method for reducing drift induced by
a changing environment feature when imaging radiation from a scene,
the radiation from the scene including at least a first and second
wavelength band in the long wave infrared region of the
electromagnetic spectrum, the method comprising: (a) focusing
radiation from the scene by a first and second substantially
wedge-shaped component through an image forming optical component
onto a detector sensitive to radiation in the first and second
wavelength bands, the detector being uncooled and including a
separate first, second, and third detector region, the first and
second wedge-shaped components positioned at a distance from the
image forming optical component such that the radiation is imaged
separately onto the first and second detector regions through an
f-number less than approximately 1.5, and each of the wedge-shaped
components transmitting radiation substantially in one respective
wavelength band, and the imaged radiation on each of the first and
second detector regions including radiation in one respective
wavelength band, the imaged radiation on the first and second
detector regions producing at least a first pixel signal; (b)
projecting radiation from a radiation source by at least one of the
first or second wedge-shaped components through the image forming
optical component onto the third detector region to produce a
second pixel signal, the radiation source different from the scene,
and the radiation source projected continuously onto the third
detector region over the duration for which the radiation from the
scene is focused onto the first and second detector regions; and
(c) modifying the first pixel signal based in part on a
predetermined function to produce a modified pixel signal, the
predetermined function defining a relationship between a change in
the second pixel signal and a change in the first pixel signal
induced by the changing environment feature.
Optionally, the image forming optical component and the first and
second wedge-shaped components are positioned within a first
enclosure volume, and the method further comprises: (d) positioning
the radiation source proximate to the first enclosure volume.
Optionally, the image forming optical component and the first and
second wedge-shaped components are positioned within a first
enclosure volume, and at least a portion of the first enclosure
volume is positioned within a second enclosure volume, and the
method further comprises: (d) positioning the radiation source
within the second enclosure volume and outside of the first
enclosure volume.
Optionally, the method further comprises: (d) determining the
change in the first pixel signal induced by the changing
environment feature based on the predetermined function, and the
modified pixel signal is produced by subtracting the determined
change in the first pixel signal from the first pixel signal.
Optionally, the predetermined function is a correlation between the
second pixel signal and the change in the first pixel signal
induced by the changing environment feature.
Optionally, the method further comprises: (d) determining the
correlation, the determining of the correlation being performed
prior to performing (a).
Optionally, the radiation source is a blackbody radiation source,
and the detector and the first enclosure volume are positioned
within a chamber having an adjustable chamber temperature, and a
verification of the correlation is determined by: (i) measuring a
first temperature of the blackbody radiation source at a first
chamber temperature and measuring a subsequent temperature of the
blackbody radiation source at a subsequent chamber temperature, the
first and subsequent temperatures of the blackbody radiation source
defining a first set; (ii) measuring a first reading of the second
pixel signal at the first chamber temperature and measuring a
subsequent reading of the second pixel signal at the subsequent
chamber temperature, the first and subsequent readings of the pixel
signal defining a second set; and (iii) verifying a correlation
between the first and second sets.
Optionally, the radiation source is a blackbody radiation source,
and the detector and the first enclosure volume are positioned
within a chamber having an adjustable chamber temperature, and a
determination of the correlation includes: (i) measuring a first
reading of the first pixel signal at a first chamber temperature
and measuring a subsequent reading of the first pixel signal at a
subsequent chamber temperature; (ii) subtracting the first reading
of the first pixel signal from the subsequent reading of the first
pixel signal to define a first set; and (iii) measuring a first
reading of the second pixel signal at the first chamber temperature
and measuring a subsequent reading of the second pixel signal at
the subsequent chamber temperature, the first and subsequent
readings of the second pixel signal defining a second set.
Optionally, the modifying of the first pixel signal includes: (i)
measuring a first reading of the first pixel signal at a first time
instance and measuring a subsequent reading of the first pixel
signal at a subsequent time instance; (ii) measuring a first
reading of the second pixel signal at the first time instance and
measuring a subsequent reading of the second pixel signal at the
subsequent time instance; and (iii) subtracting the first reading
of the blind pixel signal from the subsequent reading of the blind
pixel signal to define a third set.
Optionally, the modifying of the first pixel signal further
includes: (iv) modifying the subsequent reading of the first pixel
signal based on the third set in accordance with a correlation
between the first and second sets.
Optionally, the determination of the correlation further includes:
(iv) displaying the first set as a function of the second set.
Optionally, the determination of the correlation further includes:
(iv) displaying the first set as a function of a third set, the
third set being defined by the first chamber temperature and the
subsequent chamber temperature.
There is also provided according to an embodiment of the teachings
of the present invention, a device for reducing a drift induced by
a changing environment feature when imaging radiation from a scene,
the radiation from the scene including at least a first and second
wavelength band in the long wave infrared region of the
electromagnetic spectrum, the device comprising: (a) a radiation
source, the radiation source different from the scene; (b) a
detector of the radiation from the scene and of radiation from the
radiation source, the detector being uncooled and sensitive to
radiation in the first and second wavelength bands, and the
detector including a separate first, second, and third detector
region; (c) a first and a second filter, the first filter
associated with the first detector region for allowing radiation in
the first wavelength band to be imaged on the first detector
region, the second filter associated with the second detector
region for allowing radiation in the second wavelength band to be
imaged on the second detector region; (d) an optical system for
continuously focusing the radiation from the scene and the
radiation source onto the detector, the optical system comprising:
(i) an image forming optical component for forming an image of the
scene on the detector and for projecting radiation from the
radiation source onto the third detector region, and (ii) a first
and a second substantially wedge-shaped component, the first
wedge-shaped component associated with the first filter, the second
wedge-shaped component associated with the second filter, each of
the wedge-shaped components fixedly positioned at a distance from
the image forming optical component, each of the wedge-shaped
components directing radiation from a field of view of the scene
through the image forming optical component onto the detector, such
that the radiation is imaged separately onto the first and second
detector regions through an f-number of the optical system of less
than approximately 1.5, the imaged radiation on each of the
detector regions including radiation in one respective wavelength
band, and at least one of the first or second wedge-shaped
components projecting radiation from the radiation source through
the image forming optical component onto the third detector region;
and the device further comprising (e) electronic circuitry
configured to: (i) produce at least a first pixel signal from the
imaged radiation on the first and second detector regions; (ii)
produce a second pixel signal from the radiation source projected
by the optical system onto the third detector region, and (iii)
modify the first pixel signal according to a predetermined function
to produce a modified pixel signal, the predetermined function
defining a relationship between a change in the second pixel signal
and a change in the first pixel signal induced by the changing
environment feature.
Optionally, the electronic circuitry is further configured to: (iv)
determine the change in the first pixel signal induced by the
changing environment feature based on the predetermined function;
and (v) subtract the determined change in the first pixel signal
from the first pixel signal.
Optionally, the radiation source is a blackbody radiation
source.
Optionally, the radiation from the radiation source is directed by
only one of the first and second wedge-shaped components through
the image forming optical component onto the third detector
region.
Optionally, the device further comprises: (f) a first enclosure
volume, the optical system being positioned within the first
enclosure volume.
Optionally, the radiation source is positioned proximate to the
first enclosure volume.
Optionally, the device further comprises: (g) a second enclosure
volume, at least a portion of the first enclosure volume being
positioned within the second enclosure volume, and the radiation
source being positioned within the second enclosure volume and
outside of the first enclosure volume.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention is herein described, by way of example only, with
reference to the accompanying drawings, wherein:
FIG. 1 is a schematic side view illustrating a device for imaging
radiation from a scene in two wavelength regions with no moving
parts;
FIG. 2 is a schematic side view illustrating the traversal of
incident rays from the scene through the device of FIG. 1;
FIG. 3 is a schematic side view illustrating a device for drift
correction according to an embodiment of the invention;
FIG. 4A is a schematic front view illustrating a detector array of
the device of FIG. 1;
FIG. 4B is a schematic front view illustrating a detector array of
the device of FIG. 3;
FIG. 4C is a schematic front view illustrating blind pixels and
imaged pixels according to an embodiment of the invention;
FIG. 5 is a block diagram of image acquisition electronics coupled
to a detector array according to an embodiment of the
invention;
FIG. 6 is a flowchart for verifying a correlation according to an
embodiment of the invention;
FIG. 7 is a flowchart for determining a correlation according to an
embodiment of the invention;
FIG. 8 is flowchart for correcting for drift according to an
embodiment of the invention.
FIGS. 9A and 9B show examples of plots used for performing steps of
the flowchart of FIG. 6
DESCRIPTION OF THE PREFERRED EMBODIMENTS
The present invention is a method and device for providing a
functionality for drift correction in an infrared dual band imaging
system as described below, which does not use moving parts.
The principles and operation of the method and device according to
the present invention may be better understood with reference to
the drawings and the accompanying description.
The present invention is applicable to infrared imaging devices and
systems for imaging a scene in two wavelength regions of the
infrared spectral range, most preferably the Long-Wave Infrared
(LWIR) region of the electromagnetic spectrum, by using a pair of
stationary wedge-shaped optical components. The particular value of
the present invention rests in providing a means to ensure
quantitative results of a gas distribution in the scene, by
compensating for signal drifts without using moving parts.
With reference to the drawings, a schematic illustration of an
example of such a device 1 for imaging radiation from a scene 80 is
shown in FIG. 1. When imaging such a scene, the device 1 is
positioned such that the scene 80 is interposed between the device
1 and a radiation emitting background 90, such as, for example, a
collection of objects (such as pipes and walls), the horizon, the
sky or any other suitable background. Infrared radiation in at
least two wavelength bands, a first wavelength band .lamda..sub.G
and a second wavelength band .lamda..sub.N, is emitted from the
background. The characteristics of the scene 80 are such that it is
absorbent, at least in part, and emitting of radiation in one of
the wavelength bands and non-absorbent (and therefore non-emitting)
of radiation in the other wavelength band. For example, the scene
80 may be absorbent and emitting of radiation in the first
wavelength band (.lamda..sub.G) and non-absorbent and non-emitting
of radiation in the second wavelength band (.lamda..sub.N). As a
result, data acquired through a filter approximately centered about
.lamda..sub.G carries information about both the gas presence and
the background emission. Similarly, data acquired through a filter
approximately centered about .lamda..sub.N carries information
about the background emission, but does not carry information about
the gas presence. Special algorithms subsequently extract relevant
gas cloud information from the acquired data in the two wavelength
bands.
The imaging itself is done by an infrared detector array 14 that
includes two separate regions, a first detector region 14a and a
second detector region 14b. The detector array 14 is positioned
within a detector case 12, in turn positioned within the device 1.
Each of the detector regions includes a plurality of detector
elements (not shown) corresponding to individual pixels of the
imaged scene. Although the image acquisition electronics associated
with the detector array 14 are not shown in FIG. 1, it should be
understood that the image acquisition electronics includes
electronic circuitry that produces corresponding imaged pixel
signals for each pixel associated with a detector element. As a
result of the radiation being imaged on a plurality of detector
elements, the image acquisition electronics produces a plurality of
imaged pixel signals.
The present invention specifically addresses systems based on an
uncooled detector array 14 such as, for example, a microbolometer
type array.
Radiation from the scene 80 and the background 90 is focused onto
the detector array 14 through a window 16 by an optical system 18
whose optical components are represented symbolically in FIG. 1 by
an objective lens 20 and first and second wedge-shaped components
22 and 24. Note that the "objective lens" 20 may actually be a set
of one or more lenses that is represented in FIG. 1 by a single
lens. The optical system 18 can be considered as a first enclosure
volume for maintaining the position and the orientation of the
optical components. The device 1 can be considered as a second
enclosure volume, defined by internal walls 30, for maintaining the
position and orientation of the optical system 18 and the detector
array 14.
The same infrared radiation from the scene 80 is imaged onto each
of the two detector regions 14a and 14b, with each region of the
detector imaging the scene 80 in a different wavelength band. The
traversal of incident rays 42a-42f and 44a-44f from the scene 80 to
the detector array 14 is shown in FIG. 2. The objective lens 20
focuses radiation deflected by the wedge-shaped components 22 and
24 on the detector array 14 to form two simultaneous and separate
images of the scene 80 with the background 90, each image being
formed on one half of the detector surface. As such, the radiation
from the scene 80 and its background 90 is imaged separately and
simultaneously onto the detector regions 14a and 14b.
The scene 80 and the background 90 is imaged by the device 1 with
no moving parts while maintaining a high numerical aperture and low
f-number (f/1.5 or less) at the detector array 14. This is
accomplished by positioning each of the first and second
wedge-shaped components 22 and 24 at a minimum fixed distance d
from the objective lens 20 along the optical axis of the device 1.
Positioning the wedge-shaped components 22 and 24 at a sufficiently
large enough distance from the objective lens 20, in combination
with the above mentioned deflection angles, allows for the low
f-number (high numerical aperture) at the detector array 14 to be
maintained. This corresponds to high optical throughput of the
device 10. As a result, the same radiation from the scene is
deflected by the wedge-shaped components 22 and 24 toward the
objective lens 20 and imaged on the detector regions 14a and 14b
through an f-number of the optical system 18 which can be
maintained close to 1 (f/1) without having to decrease the focal
length for increase the aperture diameter D. Accordingly, the
minimum distance d which provides such high optical throughput can
be approximately lower bounded by:
>.times..times..function..theta. ##EQU00001##
Having a large numerical aperture (low f-number) provides higher
sensitivity of the detector array 14 to the radiation from the
scene 80, and less sensitivity to radiation originating from within
the internal walls 30 of the device 1, the optical system 18, and
the optical components themselves.
As a result of positioning the wedge-shaped components 22 and 24 at
the distance d, the vertical fields of view of the wedge-shaped
components 22 and 24 are approximately half of the above mentioned
vertical field of view of the objective lens 20.
The wedge-shaped components 22 and 24 are preferably positioned
symmetrically about the optical axis, such that each is positioned
at the same distance d from the objective lens 20, and each is
positioned at the same angle relative to the optical axis. Such a
design ensures that the same amount of radiation is imaged on the
detector regions 14a and 14b via the objective lens 20 from the
wedge-shaped components 22 and 24.
As previously mentioned, the radiation from the scene 80 which is
imaged onto the first detector region 14a only includes one of the
wavelength bands. The radiation from the scene 80 which is imaged
onto the second detector region 14b only includes the other one of
the wavelength bands. This is accomplished by positioning filters
26 and 28, most preferably band pass filters, in the optical
train.
Suppose, for example, that it is desired that the radiation from
the scene 80 imaged on the first detector region 14a only includes
radiation in the first wavelength band (.lamda..sub.G), and the
radiation from the scene 80 imaged on the second detector region
14b only includes radiation in the second wavelength band
(.lamda..sub.N). Accordingly, the first filter 26 filters radiation
in spectral ranges outside of the first wavelength band
(.lamda..sub.G) and the second filter 28 filters radiation in
spectral ranges outside of the second wavelength band
(.lamda..sub.N). Thus, the radiation from the scene 80 that is
directed by the first wedge-shaped component 22 to be imaged on the
first detector region 14a includes only radiation in the first
wavelength band (.lamda..sub.G), and the radiation from the scene
80 that is directed by the second wedge-shaped component 24 to be
imaged on the second detector region 14b includes only radiation in
the second wavelength band (.lamda..sub.N).
The surface of the detector array 14 is divided into the two
aforementioned regions by a dividing line 32 as shown in FIG. 4A.
FIG. 1 includes a non-limiting exemplary representation of the
Cartesian coordinate system XYZ in which the detector plane is
parallel to the YZ plane. Accordingly, the dividing line 32 is
parallel to the Z axis and the optical axis is parallel to the
X-axis. The wedge-shaped components 22 and 24 are wedge-shaped in
the XY plane.
As previously discussed, the large numerical aperture and low
f-number provides higher sensitivity of the detector array 14 to
the radiation from the scene 80. However, changes in the
environmental temperature surrounding the device 1 causes the
emission of radiation originating from within the internal walls 30
of the imaging device 1, the optical system 18, and the optical
components themselves to vary with time, which in turn leads to
drifts in the imaged pixels signals, and erroneous results in the
gas path concentration of each pixel of the image of the scene as
measured by the device 1 according to appropriate algorithms.
Refer now to FIG. 3, a device 10 for reducing the effect of the
unwanted radiation according to an embodiment of the present
disclosure. The description of the structure and operation of the
device 10 is generally similar to that of the device 1 unless
expressly stated otherwise, and will be understood by analogy
thereto. Ideally, the device 10 reduces the signal drift to a
negligible amount essentially correcting for the effect of the
drift. Accordingly, the terms "correcting for", "compensating for"
and "reducing", when applied to drift in imaged pixels signals, are
used interchangeably herein.
For simplicity and disambiguation, the device 10 is hereinafter
referred to as the imaging device 10. The term "imaging device" is
used herein to avoid confusing the device 1 with the imaging device
10, and is not intended to limit the functionality of the imaging
device 10 solely to imaging. The imaging device 10 may also include
functionality for detection, measurement, identification and other
operations relevant to infrared radiation emanating from a
scene.
A specific feature of the imaging device 10 which is not shown in
the device 1 is image acquisition electronics 50 associated with
the detector array 14. As shown in FIG. 3, the image acquisition
electronics 50 is electrically coupled to the detector array 14 for
processing output from the detector in order to generate and record
signals corresponding to the detector elements for imaging the
scene 80. As will be discussed, the image acquisition electronics
50 is further configured to apply a correction to the generated
scene pixels signals in order to reduce the drift in the generated
scene pixels signals caused by the radiation originating from
within the internal walls 30 of the imaging device 10, the optical
system 18, and the optical components themselves.
Refer now to FIG. 5, a block diagram of the image acquisition
electronics 50. The image acquisition electronics 50 preferably
includes an analog to digital conversion module (ADC) 52
electrically coupled to a processor 54. The processor 54 is coupled
to a storage medium 56, such as a memory or the like. The ADC 52
converts analog voltage signals from the detector elements into
digital signals. The processor 54 is configured to perform
computations and algorithms based on the digital signals received
from the ADC 52.
The processor 54 can be any number of computer processors
including, but not limited to, a microprocessor, an ASIC, a DSP, a
state machine, and a microcontroller. Such processors include, or
may be in communication with computer readable media, which stores
program code or instruction sets that, when executed by the
processor, cause the processor to perform actions. Types of
computer readable media include, but are not limited to,
electronic, optical, magnetic, or other storage or transmission
devices capable of providing a processor with computer readable
instructions.
As shown in FIG. 3, the image acquisition electronics 50 may be
positioned outside of the detector case 12. Alternatively, the
image acquisition electronics 50 may be included as part of the
detector array 14 and detector case 12 combination.
Another specific feature of the imaging device 10 that is different
from the device 1 is the partition of the detector array 14 into
separate regions. As shown in FIG. 4B, the detector array 14 of the
imaging device 10 is partitioned into three separate regions, a
first detector region 14a, a second detector region 14b, and a
third detector region 14c. The area of the third detector region
14c is significantly smaller or not usually larger than the areas
of the other two detector regions and can be visualized as a strip
extending across the center of the detector plane along the
Z-axis.
The optical system composed of the wedge-shaped components 22 and
24, and the objective lens 20 simultaneously images the scene 80
upside down in both regions 14a and 14b while projecting infrared
radiation emitted by a surface 60 (e.g. a blackbody radiation
source) onto the third detector region 14e. The surface 60 is in
good thermal contact with the internal walls 30 of the device and
is in the vicinity of the optical components, so that the
temperature of the surface 60 can be assumed to be at all times at
the temperature of the integral walls 30 and optical system 18,
which in turn is affected by (and usually, especially when used in
outdoor conditions, close to) the environment temperature. In other
words, the signals of the detector elements of the third detector
region 14c do not carry information from the scene 80, but rather
carry information on the self emitted radiation of the internal
walls 30 and optical system 18 of the device. Therefore, the pixels
signals of the third detector region 14c can be used by the device
10 algorithms and electronics to correct for the unwanted changes
to the signals of the detector regions 14a and 14b that are caused
by changing environment and not by the corresponding regions of
scene 80. The pixels of the third detector region 14c are referred
to as "blind pixels", Additionally, a baffle or baffles may be
positioned to prevent radiation from the scene 80 from reaching the
third detector region 14c.
The above explanation constitutes a third specific feature of the
imaging device 10, which is different from the device 1, namely the
inclusion of the blackbody radiation source 60 within the internal
walls 30 of the imaging device 10. The blackbody radiation source
60 is positioned such that the blackbody radiation source 60 emits
radiation which is projected only onto the third detector region
14c, resulting in the blind pixels as previously mentioned to
produce signals which, as will be discussed in more detail below,
are used to reduce the drift in the signals from the scene, due to
changing case and optics self-emission. The traversal of incident
rays 64a and 64b from the blackbody radiation source 60 to the
detector array 14 is shown in FIG. 3. Note that the traversal of
the rays as depicted in FIG. 3 is not drawn to scale (due to
drawings space constraints), and that the deflection angle between
the rays 64a and 64b is approximately the same as the deflection
angle between the rays 44d and 44e (FIG. 2).
The blackbody radiation source 60 can be placed in various
positions within the imaging device 10. Preferably, the blackbody
radiation source 60 is placed in contact with the internal walls 30
of the imaging device 10 and outside of the optical system 18, and
most preferably in proximity to the optical system 18. The
placement of the blackbody radiation source 60 within the imaging
device 10 is incumbent upon the radiation being focused by the
optical system 18 onto only the third detector region 14c to
generate the blind pixels signals.
In the non-limiting implementation of the imaging device 10 shown
in FIG. 3, the blackbody radiation source 60 is positioned such
that the radiation from the blackbody radiation source 60 is
directed by the second wedge-shaped component 24 through the
objective lens 20 onto the third detector region 14c. Note that in
addition to the blackbody radiation source 60, an additional
blackbody radiation source 70 can be placed in a symmetric position
about the X-axis such that the radiation from the blackbody
radiation source 70 is directed by the first wedge-shaped component
22 through the objective lens 20 onto the third detector region 14c
as well.
The process of reducing and/or correcting for the drift in the
generated scene pixels signals is applied to all scene pixels
signals. For clarity, the process will be explained with reference
to correcting for the drift in a single scene pixel signal.
The optical components, the optical system 18, and the spaces
between the internal walls 30 are assumed to be at a temperature
T.sub.E, which is usually close to and affected by the temperature
of the environment in which the imaging device 10 operates. As a
result, the amount of radiation originating from the optical
components and the optical system 18 is a direct function of the
temperature T.sub.E.
Since the blackbody radiation source 60 (and 70 if present) is
placed within the imaging device 10 and in good thermal contact
with the device 10, the optical system 18 and the internal walls
30, the temperature of the blackbody radiation source 60 (T.sub.BB)
is assumed to be the same or a function of the temperature T.sub.E
(i.e. T.sub.BB and T.sub.E are correlated). T.sub.B8 can be
measured by a temperature probe 62 placed in proximity to, or
within, the blackbody radiation source 60.
A measured scene pixel signal S from a region of the scene, can be
expressed as the sum of two signal terms, a first signal term
S.sub.O, and a second signal term S.sub.S. The first signal term
S.sub.O is the signal contribution to S corresponding to the
radiation originating from the optical components, the optical
system 18, and internal walls 30 of the device 10. The second
signal term S.sub.S is the signal contribution to S due to the
radiation originating from the corresponding region of the scene 80
imaged on the pixel in question. Accordingly, the scene pixel
signal S is the result of the combination of radiation originating
from the internal walls 30 and environment, optical components and
the optical system 18, and radiation from the scene 80, being
imaged onto the two detector regions 14a and 14b.
Since the blackbody radiation source 60 is assumed to be at a
temperature that is a direct function of the temperature T.sub.E,
the radiation emitted by the blackbody radiation source 60 is
representative of the radiation originating from the optical
components and the optical system 18 and internal walls 30 and
environment. Accordingly, a blind pixel signal, S.sub.B, may be
assumed to be also a good representation of the contribution to the
scene pixel signal due to the radiation originating from the
environment, the optical components and the optical system 18.
As a result of the radiation originating from the optical
components and the optical system 18 being a direct function of the
temperature T.sub.E, the first signal term S.sub.O (if the above
assumptions are correct) is also a direct function of the
temperature T.sub.E. This can be expressed mathematically as
SO=f1(TE), where f1() is a function.
Similarly, as a result of the blind pixel signal S.sub.B being
assumed to be a good representation of the pixel signal
contribution corresponding to the radiation originating from the
optical components and the optical system 18, the blind pixel
signal S.sub.B can also be assumed to be a direct function of the
internal walls 30, the environment and optical system temperature
T.sub.E. This can be expressed mathematically as
S.sub.B=f.sub.2(T.sub.E), where f.sub.2() is also a function.
Accordingly, since both the first signal term S.sub.O and the blind
pixel signal S.sub.B are functions of the same operating
temperature T.sub.E, it is conceivable that a correlation may exist
between the first signal term S.sub.O and the blind pixel signal
S.sub.B. With the knowledge of the correlation (if existing), the
first signal term S.sub.O and the changes in time of S.sub.O
(referred to hereinafter as "scene pixel signal drifts") can be
determined from the blind pixel signal S.sub.B and the changes in
time of S.sub.B. Accordingly, in the above assumptions, the changes
in time or drifts of the scene pixel signal S due to environment
status can be removed and corrected for, in order to prevent gas
quantity calculation errors.
In the context of this document, the term "correlation", when
applied to a relationship between sets of variables or entities,
generally refers to a one-to-one relationship between the sets of
variables. As such, a correlation between the first signal term
S.sub.O and the blind pixel signal S.sub.B indicates a one-to-one
relationship between the first signal term S.sub.O and the blind
pixel signal S.sub.B at any temperature of the imaging device 10.
This correlation is determined by a sequence of controlled
measurements. The sequence of controlled measurements is performed
prior to when the imaging device 10 is in operation in the field,
and can be considered as a calibration procedure or process to be
performed in manufacturing of the device. For the purposes of this
document, the imaging device 10 is considered to be in an
operational stage when the radiation from the scene 80 is imaged by
the detector array 14 and the drift in the generated imaged pixels
signals is actively reduced by the techniques as will later be
described.
Recall the assumption that the blackbody radiation source 60 is at
a temperature that is a direct function of the temperature T.sub.E.
According to this assumption, the blind pixel signal S.sub.B is
assumed to be a good representation of the pixel signal
contribution due to the radiation originating from the optical
components and the optical system 18. Prior to determining the
correlation function between the first signal term S.sub.O and the
blind pixel signal S.sub.B, it is first necessary to verify the
actuality of the above assumptions. Subsequent to the verification,
the correlation function between the time changes of the first
signal term S.sub.O (scene pixel signal drifts) and the blind pixel
signal S.sub.B time changes can be determined. Both the
verification process, and the process of determining the
correlation function, is typically conducted through experiment. In
practice, only drifts, or unwanted changes of the imaged pixel
signals over time are to be corrected for, so the process of
verification and determination of the correlations are only needed
and performed between the differentials of S.sub.O, S.sub.B, or
variations during time due to environment temperature
variations.
Refer now to FIG. 6, a flowchart of a process 600 for verifying the
existence of a correlation between the environment temperature, the
temperature of the blackbody radiation source 60 (and 70 if
present) and the blind pixel signal S.sub.B. In step 601, the
imaging device 10 is placed in a temperature controlled
environment, such as a temperature chamber having a controllable
and adjustable temperature, and to point the device 10 at an
external blackbody source at a fixed temperature T.sub.F so that
the scene pixels of the detector regions 14a and 14b are exposed to
unchanging radiation from the external blackbody. Such an external
blackbody source is used in place of the scene 80 depicted in FIGS.
2 and 3. In step 602, the temperature of the temperature chamber is
set to an initial temperature T.sub.0. The temperature of the
temperature chamber and the imaging device 10 are let to stabilize
to temperatures T.sub.0 and T.sub.E respectively by allowing for an
appropriate interval of time to pass.
Once the temperatures have stabilized, T.sub.BB (which may be
practically equal to T.sub.E) is measured via the temperature probe
62 in step 604. In step 606, the blind pixel signal S.sub.B is
measured via the image acquisition electronics 50. Accordingly, the
blind pixel signal S.sub.B and T.sub.BB are measured at temperature
T.sub.0 in steps 604 and 606, respectively.
In step 608, the temperature of the temperature chamber is set to a
different temperature T.sub.1. Similar to step 602, the
temperatures of the temperature chamber and the imaging device 10
are let to stabilize to temperature T.sub.1 and a new temperature
T.sub.E, respectively, by allowing for an appropriate interval of
time to pass. Once the temperatures have stabilized, T.sub.BB is
measured via the temperature probe 62 in step 610. In step 612, the
blind pixel signal S.sub.B is measured via the image acquisition
electronics 50. Accordingly, the blind pixel signal S.sub.B and
T.sub.BB are measured at chamber temperature T.sub.1 in steps 610
and 612, respectively.
The process may continue over a range of chamber temperatures of
interest, shown by the decision step 613. For each selected chamber
temperature, the blind pixel signal S.sub.B and T.sub.BB and
T.sub.E are measured as in steps 604, 606, 610 and 612 above.
In step 614, the existence of a correlation between the environment
temperature, the blind pixel signal S.sub.B and the temperature of
the blackbody radiation source 60 (and 70 if present) is verified
by analyzing the resultant measurements. For example, the blind
pixel signal S.sub.B measurements from steps 604 and 610 can be
plotted as function of the operating temperatures T.sub.E
established in steps 602 and 608. Similarly, the T.sub.BB
measurements from steps 606 and 612 can be plotted or otherwise
visualized versus the range of operating temperatures T.sub.E
established in steps 602 and 608. An example of plots for executing
step 614 is depicted in FIGS. 9A and 9B.
Referring first to FIG. 9A, an example of plots of the measurements
of the operating temperatures (T.sub.E), the blind pixel signal
(S.sub.B), and the blackbody radiation source temperature (T.sub.BB
measured via the temperature probe 62) is depicted. The plots shown
in FIG. 9A are intended to serve as illustrative examples, and
should not be taken as limiting in the scope or implementation of
the process 600.
Note that the x-axis in FIG. 9A is designated as "time (t)", as
should be apparent due to the variation of the operating
temperatures (T.sub.E) as time (t) goes by. Also note that the
example plots shown in FIG. 9A includes two y-axes. The first
y-axis (shown on the left side of FIG. 9A) is designated as
"temperature" and corresponds to the operating temperatures
(T.sub.E) and the blackbody radiation source temperature
(T.sub.BB). The second y-axis (shown on the right side of FIG. 9A),
is designated as "signal counts" and is the measured output of the
ADC 52 corresponding to the blind pixel signal (S.sub.B).
If there is a linear (or any other one-to-one) relationship between
the three entities T.sub.E, T.sub.BB, and S.sub.B, the above
discussed assumptions are upheld to be valid, and therefore there
exists a correlation between the temperatures T.sub.E, T.sub.BB,
and the blind pixel signal S.sub.B.
Referring now to FIG. 9B, the recognition of such a linear
relationship can be shown by alternatively plotting the
measurements depicted in FIG. 9A. As should be apparent, the
example plots shown in FIG. 9B show the blackbody radiation source
temperature (T.sub.BB) and the blind pixel signal (S.sub.B) signal
counts versus the temperature T.sub.E, which, as previously
discussed, is the environment temperature. Accordingly, the x-axis
in FIG. 9B is designated as "environment temperature". As in FIG.
9A, FIG. 9B also includes two y-axes. The first y-axis (shown on
the left side of FIG. 9B) is designated as "temperature" and
corresponds to the blackbody radiation source temperature
(T.sub.BB). The second y-axis (shown on the right side of FIG. 9B),
is designated as "signal counts" and is the measured output of the
ADC 52 corresponding to the blind pixel signal (S.sub.B).
Similar to the plots shown in FIG. 9A, the plots shown in FIG. 9B
are intended to serve as illustrative examples, and should not be
taken as limiting in the scope or implementation of the process
600. As can be clearly seen in the illustrative example depicted in
FIG. 9B, a linear relationship of non-zero slope (which is an
example of a one-to-one relationship) exists between the three
entities T.sub.E, T.sub.BB, and S.sub.B, thus implying that the
three entities are correlated.
Refer now to FIG. 7, a flowchart of a process 700 for determining a
correlation between the drifts of scene pixels signals and the
blind pixel signals S.sub.B changes due to changes in environment
temperature. Similar to the process 600, before performing the
process 700, the imaging device 10 is placed in the temperature
chamber. The imaging device 10 is also pointed at a source of
infrared radiation representing and simulating a scene during the
operation of the device 10, most conveniently a blackbody source at
a known and fixed temperature. The blackbody may be positioned
inside the temperature chamber or outside of the temperature
chamber and measured by the imaging device 10 through an infrared
transparent window. In the process 700, measurements of the scene
pixel signal S and the blind pixel signal S.sub.B are made via the
image acquisition electronics 50.
In step 701 (similar to step 601 above), the device 10 is retained
in the temperature chamber and pointed at the external blackbody
source which is set to a fixed temperature T.sub.F. In step 702,
the temperature of the temperature chamber is set to an initial
temperature T.sub.0. The chamber and the device 10 are let to
stabilize at temperature T.sub.0 by waiting an appropriate period
of time. In step 704, the imaged pixel signal S and the blind pixel
signal S.sub.B are measured after the temperature of the imaging
device 10 reaches stabilization at T.sub.0.
In step 706, the temperature of the temperature chamber is set to a
new temperature T.sub.1, and the external blackbody is maintained
at the temperature T. The chamber and the device 10 are let to
stabilize at temperature T.sub.1 by waiting an appropriate period
of time. In step 708, the scene pixel signal S and the blind pixel
signal S.sub.B are measured after the temperature of the imaging
device 10 reaches stabilization at T.sub.1.
In step 710, the imaged pixel signal S measured in step 704 is
subtracted from the imaged pixel signal S measured in step 708. The
result of step 710 yields the temporal drift of the imaged pixel
signal due to the change in the temperature of the temperature
chamber. Also in step 710, the blind pixel signal S.sub.B measured
in step 704 is subtracted from the blind pixel signal S.sub.B
measured in step 708.
Similar to the process 600, the process 700 may continue over a
range of chamber temperatures of interest, shown by decision step
712. For each selected chamber temperature, the imaged pixel signal
S measured in step 704 is subtracted from the imaged pixel signal S
measured at the selected temperature, and the blind pixel signal
S.sub.B measured at step 704 is subtracted from the blind pixel
signal S.sub.B measured at the selected temperature. This procedure
can be performed for all the operating temperature ranges of the
imaging device.
In step 714, the resultant differences in the scene pixels obtained
in step 710 are plotted as function of the blind pixel differences
obtained at each chamber temperature. In step 716, the correlation
function is determined by analyzing the results of the plot
obtained in step 714. Numerical methods, such as, for example,
curve-fitting, least-squares, or other suitable methods, can be
used to further facilitate the determination of the correlation
function.
As should be apparent, the resulting correlation function can be
interpolated and extrapolated to cover operating temperature ranges
not measured during the execution of the processes 600 and 700. In
step 718, the correlation function determined in step 716 is stored
in a memory coupled to the processor 54, such as, for example, the
storage medium 56.
Note that typical environment temperature variations used during
the execution of the processes 600 and 700 may depend on various
factors such as, for example, the location of the imaging device 10
when in the operational stage and the intended specific use of the
imaging device 10 when in the operational stage. For example, when
the imaging device 10 is used for monitoring in industrial
installations and facilities for gas leakages, the temperature
variations occurring during the execution of the processes 600 and
700 are typically in the range of tens of degrees.
As a result of the correlation function determined by the process
700, during the operation of the imaging device 10, signal drifts
of the measured scene pixel signals can be compensated in real time
while the temperature of the environment changes. The process of
compensating and/or correcting for the signal drifts during
operation of the imaging device 10 is detailed in FIG. 8.
Refer now to FIG. 8, a flowchart of a process 800 for correcting
for the signal drifts in the imaged pixel signal S caused by
environment temperature changes, while the device 10 is operational
in the field. In steps 802-814 the device 10 is operational in the
field and monitors a scene in an industrial environment,
automatically and without human intervention.
In step 802, the scene pixel signal S is measured and stored at an
initial time t.sub.0. The scene pixel measured at time t.sub.0 may
be stored in the storage medium 56 or stored in a temporary memory
coupled to the processor 54. In step 804, the blind pixel signal
S.sub.B is measured at the same initial time t.sub.0. In step 806,
the scene pixel signal S is measured at a subsequent time t.sub.S
after the initial time t.sub.0. In step 808, the blind pixel signal
S.sub.B is measured at the same subsequent time t.sub.S.
In step 810, the blind pixel signal S.sub.B measured in step 804 is
subtracted from the blind pixel signal S.sub.B measured in step
808. In step 810, the drift of scene pixel signal that occurred
between the measurement time t.sub.0 and t.sub.S (due to change in
the environment temperature) is determined from the correlation
function of signal differences determined and stored in the
procedure 700. The determination of the drift of scene pixel signal
in step 810 is accomplished by subtracting the blind pixel signal
measured in step 804 from the blind pixel signal measured in step
808. The resultant difference in blind pixel signal measurements is
substituted into the correlation function of signal differences
determined in the procedure 700 to determine the drift of scene
pixel signal.
In step 812, the scene pixel signal S measured at step 806 is
modified by subtracting from it the drift value determined in step
810.
In step 814, the scene pixel signal modified in step 812 is used to
assess the presence or absence of the gas of interest in the
corresponding scene region, and to calculate the gas path
concentration if the gas is present. As should be apparent, steps
806-814 can be repeated, as needed, for additional measurements by
the device 10 of the scene pixel signals for the detection and path
concentration of the gas. This is shown by decision step 816.
Accordingly, if additional scene pixel signal measurements are
needed, the process 800 returns to step 806 (at a new subsequent
time t.sub.S). If no additional scene pixel signal measurements are
needed, the process ends at step 818.
Note that as a result of the structure and operation of the device
10 when in the operational stage, the radiation from the blackbody
source 60 (and 70 if present) is projected onto the third detector
region 14c continuously over the duration for which the radiation
from the scene 80 is focused onto the detector regions 14a and 14b.
This is required by the process and results in the reduced
frequency of shutter open and closing when in the operational
stage, and in a more accurate determination and quantification of
the relevant gas present in the scene.
Note that the blind pixel signal that used to correct the drift in
an imaged pixel signal is typically, and preferably, the blind
pixel signal associated with the blind pixel that is positioned
above or below the associated imaged pixel. In other words, the
blind pixel signal used to correct the drift in an imaged pixel
signal is preferably the blind pixel signal associated with the
detector element closest in position to the detector element
associated with the imaged pixel signal. For example, as shown in
FIG. 4C, the blind pixel 14c-1 is used to correct for the drift in
imaged pixel 14b-1. Likewise, the blind pixel 14c-2 is used to
correct for the drift in imaged pixel 14a-1.
As mentioned above, the above described processes 600, 700 and 800
were explained with reference to correcting for the drift in a
single imaged pixel signal. As previously mentioned, the same
processes may be performed for each of the imaged pixels signals,
and may be performed in parallel. The process for correcting for
the drift may be supplemented by known methods, such as, for
example, NUC, in order to further reduce and correct for the effect
of the signal drift. As a result of the drift correction via the
processes 600, 700 and 800 described above, the supplemental NUC
method is performed at a reduced frequency. The frequency of
operation of the supplemental NUC method is typically in the range
of once per hour to once per day.
It will be appreciated that the above descriptions are intended
only to serve as examples, and that many other embodiments are
possible within the scope of the present invention as defined in
the appended claims.
* * * * *